High-breakdown-voltage semiconductor device

Abstract

A high-breakdown-voltage semiconductor device has a first offset layer and a second offset layer the dosage of which is higher than that of the first offset layer. When the gate is in the ON state, the first offset layer functions as a resurf layer. When the gate is in the OFF state, part of the charge in the first offset layer is neutralized by a drain current flowing through an element having a low ON-resistance, however, the second offset layer functions as a resurf layer. When the drain current is [Acm −1 ], the amount of charge of electrons is q[C], and the drift speed of carriers is υ drift [cms −1 ], the dosage n 2 of the second offset layer is given by n 2 ≧I D /(q υ drift ) [cms −2 ].

What is claimed is:  
     
       1. A high-breakdown-voltage semiconductor device comprising: 
       a semiconductor substrate;  
       a body layer of a first conductivity type selectively formed on a region of a surface of the semiconductor substrate;  
       a source layer of a second conductivity type selectively formed in a surface of the body layer;  
       a drain layer of the second conductivity type selectively formed on the surface of the semiconductor substrate apart from the body layer;  
       a second layer of the second conductivity type formed on the surface of the substrate between the body layer and the drain layer, the second layer being apart from the body layer;  
       a source electrode formed to contact a surface of the body layer and a surface of the source layer; and  
       a gate electrode formed on at least a region of the body layer located between the source layer and the drain layer, the gate electrode being apart from the drain layer and the second layer, with an insulating film interposed between the gate electrode and the region of the body layer;  
       wherein, under conditions that a voltage is applied between the drain layer and the body layer, and a gate voltage having the same polarity as a drain voltage to be applied to the drain layer, within a rated gate voltage range, the rated gate voltage being a gate voltage range applicable to the gate electrode in a normal operation, and over a threshold voltage of the semiconductor device, is applied to the gate electrode, a breakdown voltage between the drain layer and the body layer is larger than a half of a breakdown voltage between the drain layer and the body layer with the gate voltage set at 0 volts.  
     
     
       2. The high-breakdown voltage semiconductor device according to  claim 1 , wherein, under conditions that the voltage is applied between the drain layer and the body layer, and the gate voltage having the same polarity as the drain voltage to be applied to the drain layer, within the rated gate voltage range and over the threshold voltage of the semiconductor device, is applied to the gate electrode, the breakdown voltage between the drain layer and the body layer is larger than 80% of the breakdown voltage between the drain layer and the body layer with the gate voltage set at 0 volts. 
     
     
       3. The high-breakdown voltage semiconductor device according to  claim 1 , wherein, under conditions that the voltage is applied between the drain layer and the body layer, and the gate voltage having the same polarity as the drain voltage to be applied to the drain layer, within the rated gate voltage range and over the threshold voltage of the semiconductor device, is applied to the gate electrode, the breakdown voltage between the drain layer and the body layer is larger than a rated voltage of the breakdown voltage between the drain layer and the body layer with the gate voltage set at 0 volts. 
     
     
       4. The high-breakdown voltage semiconductor device according to  claim 1 , wherein the semiconductor substrate is of the second conductivity type. 
     
     
       5. The high-breakdown voltage semiconductor device according to  claim 1 , when the semiconductor substrate is of the first conductivity type, further comprising a first layer of the second conductivity type on the surface of the substrate between the body layer and the second layer, and contacting the second layer, an impurity concentration of the first layer being lower than that of the second layer.